1. Field of the Invention
This invention relates to a vacuum switch having a spring loaded mechanism for closing a pair of electrical contacts, wherein swingable arms are provided for absorbing a portion of the kinetic energy of the mechanism during closing of the contacts so that deformation of shafts supporting the contacts is avoided and the tendency of the contacts to bounce is suppressed. Preferably, a first energy-absorbing arm which is initially engaged by the operating mechanism to reduce the velocity of a movable contact supporting shaft swings to impact a pair of second energy-absorbing arms which function to absorb kinetic energy from the first arm so that subsequent rebound of the latter with the operating mechanism occurs with a reduced impact force and tendency of the contacts to separate as a result of the rebounding impact is correspondingly minimized.
2. Description of the Prior Art
Vacuum interrupter switches are widely used in the power distribution industry for isolating electrical circuitry downstream of the switches for maintenance, repair, or other servicing operations. Vacuum switchgear normally includes a plurality of vacuum bottles each of which encloses a pair of separable contacts, and one of the contacts is mounted on a stationary shaft while the other contact is mounted on a shaft which is shiftable to move the associated contact toward and away from a circuit closing position in engagement with the stationary contact. The vacuum within the bottle minimizes arcing of the contacts when the latter are spaced a small distance apart. Additionally, it is common practice to immerse the vacuum bottle in a quantity of dielectric oil for transferring heat produced by electrical current through the switch away from the vacuum bottle and to enable the bottles to be located in close proximity to fuse structures.
Three phase switchgear normally includes three pairs of separable switch contacts that are enclosed within respective, side-by-side vacuum bottles mounted on a common support. In devices of this type, a single operating mechanism is often used to shift all of the movable contacts simultaneously and to ensure that all phases of the circuit are energized or de-energized at identical times. The switching mechanism may include a toggle member that is swingable to an over center position to enable tension springs to thereafter shift the movable shafts and the contacts supported thereby toward a position of engagement with respective stationary contacts with a motion that is positive and relatively fast.
In the past, it has been observed that the snap action toggle mechanism often used to quickly and positively close high voltage electrical contacts occasionally damages the switch by deforming the relatively soft copper contact-supporting shafts and essentially shortening the length of the same whereby the operating characteristics of the switch are drastically altered. In order to overcome any undesirable change in the operating characteristics of the switch, the shafts are sometimes work hardened by subjecting the switch to numerous opening and closing operations and thereafter the operating mechanism is readjusted for proper switch operation prior to shipping thereof. As can be appreciated, such a procedure is costly and also imposes undesirable frictional wear on the switch before the same is placed into actual use.
In order to overcome the problem of contact shaft deformation, U.S. Pat. No. 4,295,024, which is owned by the assignee of the present invention, describes the use of a switch closing mechanism which has a swingable energy absorbing member that absorbs a high proportion of the impact force developed during the switch closing sequence which would otherwise be transferred to the contact shafts. In brief, the energy absorber of U.S. Pat. No. 4,295,024 is pivotally mounted in a location proximal to the switch closing apparatus such that an engagement surface on the swingable member is disposed in the path of the closing apparatus during closing of the switch contacts so that a certain portion of the kinetic energy of the closing mechanism is transferred to the energy absorber in order to slow the velocity of the movable contact shaft immediately prior to closing and thereafter permit a relatively soft impact between the contacts.
Special considerations exist when a single spring loaded toggle mechanism is used to simultaneously move contact shafts of three vacuum bottle assemblies of a three phase distribution switch. For example, a wipe spring mechanism is often associated with each movable contact shaft to allow some degree of overlap between the mechanism and the contact shaft of each bottle and to compensate for minor differences in the distance each contact shaft must move to firmly seat the contacts together with equal pressure. Each wipe spring mechanism includes a spring interposed between a contact supporting shaft and an insulating rod or shaft that is connected to the closing mechanism, so that flexure of individual springs when the switch is closed compensates for contact erosion and biases all contact pairs together with a substantially equal force.
However, each of the three vacuum bottles in three phase switches are normally carried by a common mounting rod that is secured on opposite ends to the switch cabinet and which is comprised of an electrically insulative material. Mounting rods of this type are somewhat flexible and readily absorb a portion of the closing energy transmitted to the vacuum bottles by the spring loaded mechanism; however, the rods will later transfer a significant portion of the kinetic energy initially absorbed back to the vacuum bottles during subsequent oscillations. Moreover, the stiffness of the rod or resistance to lateral deflection varies along the length of the latter in accordance with the distance to rod support structure, so that, in effect, the spring constant for the bottle mounting means is somewhat different from bottle to bottle. Furthermore, it is to be remembered that the spring closing mechanism is a dynamic structure which invariably oscillates after switch closing due to kinetic energy that is imparted back to the mechanism from impact of the switch contacts, and which also oscillates because of reaction forces generated by the closing springs between members of the mechanism itself.
The phenomena of contact bounce arises when the kinetic energy of the movable contact and its associated shaft is not sufficiently dissipated during a switch closing operation. More particularly, when a movable contact collides with a stationary contact, the contacts will not remain permanently in mutual engagement without some measure of dissipation of the kinetic energy because it is not possible to find a common velocity for the contact interface which simultaneously satisfies the laws of conservation of energy and momentum. A portion of energy dissipation at the contact interface occurs by the above-mentioned deformation of the contact shafts although it is more desirable to transmit the energy into the supporting structure by means of stress waves for ultimate dissipation or storage in components remote from the contacts.
The problem of contact bounce becomes more apparent when it is realized that spring-loaded, snap action switch mechanisms of this type usually are converted into a substantially rigid body once the switch is closed, which facilitates transfer of shock waves through the mechanism by reducing the number of remote sites where oscillations can occur and eventially subside. Moreover, the closing force produced by the springs of the mechanism is relatively large, and may be approximately 500 pounds or more, in order to sufficiently provide momentum to a somewhat hefty member or beam which is often connected to all of the contact shafts for simultaneous closing thereof and also in order to overcome the total bias presented by the wipe springs of all three bottles. As such, relatively large impact forces are generated during switch closing and must be sufficiently dissipated in order to overcome the problem of contact bounce.
In U.S. Pat. No. 4,484,046, the closing mechanism of a vacuum load break switch is provided with a dashpot type shock absorber which suppresses secondary energy oscillations that are produced by the closing mechanism after impact of the contacts; however, such a shock absorber generally only snubs the oscillating reactions and does not satisfactorily reduce contact shaft velocity to eliminate the initial bounce of the contacts upon impact.